Method for manufacturing a nanowire structure

ABSTRACT

The present invention provides a method for aligning nanowires which can be used to fabricate devices comprising nanowires that has well-defined and controlled orientation independently on what substrate they are arranged on. The method comprises the steps of providing nanowires ( 1 ) and applying an electrical field (E) over the population of nanowires ( 1 ), whereby an electrical dipole moment of the nanowires makes them align along the electrical field (E). Preferably the nanowires are dispersed in a fluid during the steps of providing and aligning. When aligned, the nanowires can be fixated, preferably be deposition on a substrate ( 2 ). The electrical field can be utilised in the deposition. Pn-junctions or any net charge introduced in the nanowires ( 1 ) may assist in the aligning and deposition process. The method is suitable for continuous processing, e.g. in a roll-to-roll process, on practically any substrate materials and not limited to substrates suitable for particle assisted growth.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to manufacturing of nanowire devices, inparticular nanowire devices comprising nanowires aligned and protrudingin a pre-determined direction from a substrate.

BACKGROUND OF THE INVENTION

Over recent years the interest in semiconductor nanowires has increased.In comparison with conventional planar technology nanowire basedsemiconductor devices offer unique properties due to the one-dimensionalnature of the nanowires, improved flexibility in materials combinationsdue to less lattice matching restrictions and opportunities for noveldevice architectures. Suitable methods for growing semiconductornanowires are known in the art and one basic process is nanowireformation on semiconductor substrates by particle-assisted growth or theso-called VLS (vapor-liquid-solid) mechanism, which is disclosed in e.g.U.S. Pat. No. 7,335,908. Particle-assisted growth can be achieved by forinstance use of chemical beam epitaxy (CBE), metalorganic chemical vapordeposition (MOCVD), metalorganic vapor phase epitaxy (MOVPE), molecularbeam epitaxy (MBE), laser ablation and thermal evaporation methods.However, nanowire growth is not limited to VLS processes, for examplethe WO 2007/102781 shows that semiconductor nanowires may be grown onsemiconductor substrates without the use of a particle as a catalyst.Nanowires have been utilised to realise devices such as solar cells,field effect transistors, light emitting diodes, thermoelectricelements, etc which in many cases outperform conventional devices basedon planar technology.

Although having advantageous properties and performance the processingof nanowire devices was initially costly. One important breakthrough inthis respect was that methods for growing group III-V semiconductornanowires, and others, on Si-substrates has been demonstrated, which isimportant since it provides a compatibility with existing Si processingand non-affordable III-V substrates can be replaced by cheaper Sisubstrates.

When producing semiconductor nanowire devices comprising nanowires grownon a semiconductor substrate utilizing the above mentioned techniques anumber of limitations are experienced:

-   -   an MOCVD system is a complex vacuum system, which significantly        contributes to the production cost for the device;    -   growth is performed in batches, with inherent variations between        individual batches;    -   growth of a large number of nanowires over a large surface        yields variations between nanowires in the same batch;    -   the nanowires are grown on substrates which needs to withstand        temperatures of 400-700° C.; and    -   to align nanowires in the vertical direction, or any other        direction, on the semiconductor substrate requires controlled        epitaxial growth.

SUMMARY OF THE INVENTION

In view of the foregoing one object of the invention is to providealternative methods for producing nanowire semiconductor devices thatovercome the above-mentioned drawbacks of prior art. More particularly,it is an object to provide nanowires that have well-defined andcontrolled orientation independently of what substrate they are arrangedon.

Hence a method for aligning nanowires is provided. The method comprisesthe steps of providing nanowires and applying an electric field over thepopulation of nanowires, whereby an electric polarization in thenanowires makes them align along the electrical field. Preferably thenanowires are dispersed in a fluid (gas or liquid) during the steps ofproviding and aligning.

In addition to the polarization to make wires align in the electricfield, an electric dipole in the wires may be induced to provide furtherdirectionality to and to enhance the alignment. Such a dipole may beinduced by a pn-junction in the axial direction of the wire; by aSchottky diode between semiconductor and metallic sections of the wire;or by piezoelectric effects; and the effect may be enhanced byilluminating the wire during alignment, effectively inducing an opencircuit photo voltage between the ends of the wire. The magnitude ofthis light induced dipole is essentially independent of the illuminationstrength, since the open circuit voltage of a photodiode varies onlylogarithmically with illumination.

When aligned, the nanowires can be fixated, preferably in contact with asubstrate. The electrical field can be utilised to bring the nanowiresin contact with the substrate, or an opposed surface. Charged nanowiresare attracted to an oppositely charged surface in a uniform electricfield. Uncharged nanowires are attracted to regions with higher electricfields, in the case of a field gradient.

Charged wires in a field gradient will experience both effects, eitherin the opposite or in the same direction. The force due to charge on thewire depends only on the charge and the electric field strength. Theforce due to the gradient depends on the field strength the wiredimensions and on the electric polarizability. Thus, by arranging thetwo forces in opposite directions, nanowires can be classified accordingto length, size and composition. The gradient force on its own may alsobe used for classification, in which case only a difference in thatforce is used to guide wires in different directions.

Having nanowires with pn-junctions and/or illumination of the nanowireswith light of a pre-determined wavelength(s) may assist in aligning thenanowires and/or enables selective alignment of one or moresub-populations of nanowires.

The method may be performed in a continuous process, such as aroll-to-roll process wherein said population of nanowires are repeatedlyprovided and deposited in a pre-determined configuration along thesubstrate.

Thanks to the invention it is possible to produce nanowire devicescomprising aligned nanowires in a cost-efficient way and without beinglimited by the limitations of epitaxial methods.

One advantage of the present invention is that nanowires can be producedseparately from the deposition of the nanowires onto a substrate. Hencea continuous process can be used. This simplifies the manufacturing ofnanowire devices and improves the yield.

Nanowires deposited with the method described here may be alignedvertically, with only a small angle away from the normal, or with alarge spread of angles. In the latter case, the key is that the wireshave a clear preferential direction, so that the majority of the wireshave the same end toward the substrate. In the former case, the verticalalignment may be more important, and the up/down orientation less so.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein

FIG. 1 schematically illustrates alignment of a nanowire in accordancewith the invention.

FIG. 2 schematically illustrates the concept of classifying wires in anelectric field gradient. Long and thin wires are more strongly attractedtowards higher electric fields than shorter thicker ones. For chargedwires of the right dimension, the forces due to charge and to gradientcan be made to cancel.

FIG. 3 a schematically illustrates the dimensions and alignment anglesof a nanowire in an electric field.

FIG. 3 b illustrates the concepts of a nanowire oriented “up” and “down”with respect to the electric field.

FIG. 4 compares the theoretical alignment energy due to polarization(green, dashed), light induced dipole (red, solid), and their sum(purple, dotted) for wires of two different dimensions but at the sameelectric field (1000 V/cm). The alignment of the longer wire (4 b) isstronger than for the shorter one (4 a), but the light induced dipole istoo weak to be important.

FIG. 5 compares the theoretical alignment energy due to polarization(green, dashed), light induced dipole (red, solid), and their sum(purple, dotted) for wires of the same dimension but at differentelectric fields (1000 V/cm and 300 V/cm, respectively). At higher field(5 a, same as 4 b) the alignment is stronger than at lower field (5 b),but in the latter case the light induced dipole is strong enough todominate the alignment and provide a preferential direction.

FIG. 6 illustrates the alignment energy for the “up” (red, solid) and“down” (green, dashed) directions for the same wire as in 4 a at varyingelectric fields. There is a wide range of fields where the “up” energyis greater than 10 kT (i.e., to overcome the Brownian rotation energy of1 kT), but where the “down” energy is low. For very thin and long wires(not shown), there exists no region where directionality is important.

FIG. 7 schematically illustrates the regimes where wire alignment ofdifferent kinds is at play. The axes are not necessarily linear and theborders between the regimes are not to be seen as neither as sharp noras simply shaped as the drawing suggests.

FIG. 8 illustrates some of the primary ways to apply the invention.Naturally, the components can be rearranged in many ways, and indifferent sequences too complex to illustrate here. For example, wiresize classification may be used in combination with illumination atdifferent wavelengths and in series with vertical alignment toselectively deposit wires according to both size and composition.

DETAILED DESCRIPTION OF EMBODIMENTS

The method of the present invention comprises the steps of providingnanowires and applying an electrical field over the nanowires, wherebyan electrical dipole and/or dipole moment in the nanowires makes themalign along the electrical field.

In order to manufacture a structure comprising aligned nanowires, thealigned nanowires have to be fixated in aligned position. Furthermore,the nanowires are preferably connected electrically and/or optically inone or both ends. Thus, the aligned nanowires are preferably depositedonto a substrate.

The pre-fabricated nanowires may be dispersed in a fluid before applyingthe electric field, and accordingly the fluid containing the nanowirescan be applied to the substrate before applying the electrical field.

The electric field is in the following directed “upwards” if nototherwise explicitly stated in order to illustrate the principles of theinvention, however not limited to this.

The nanowires can be pre-fabricated before being provided for alignment.Semiconductor nanowires can be fabricated using one of theafore-mentioned methods where nanowires are epitaxially grown onsubstrates. After growth the nanowires are removed from their substrateand preferably dispersed in a fluid (gas or liquid). The nanowires canalso be fabricated using liquid solution-based chemistry or gas-phasesynthesis where the nanowires grow from seed particles. In theseprocesses the nanowires can remain in the remainder of the liquid orgas, respectively or be transferred to a suitable fluid, which also maybe a liquid or a gas.

Unipolar nanowires, nanowires with axial pn-junctions, nanowires withradial pn-junctions, heterostructure nanowires, etc. may be used and aregenerally fabricated using one of the above-mentioned techniques.Nanowires with axial pn-junctions are grown in a in a single process,where the seed particle contains a dopant for one polarity, and wherethe opposite polarity is achieved when the dopant is exhausted or in amore complex process, where dopants and source materials are explicitlyintroduced during the process. Nanowires with radial pn-junctions aregrown in a two-stage process, where growth conditions are changed togive radial growth, but otherwise similar to the fabrication ofnanowires with axial pn-junctions.

Nanowires may be given a net electric charge either during growth or ina separate step.

The electric dipole in the nanowires can, by way of example, beaccomplished by one or a combination of the following:

-   -   1. An electric field will induce an electric polarization in any        conducting, semiconducting or insulating nanowire, and the        nanowires will orient themselves along the electric field (FIG.        1).        -   a. For unipolar nanowires, the nanowire will be oriented            along the electrical field, but with no preferred direction            for a seed particle end.        -   b. A unipolarly doped nanowire with an axial gradient in the            doping will be preferentially oriented, since the more            highly p(n)-doped end will be more easily charged positively            (negatively), directing this end up (down) in the electric            field.    -   2. A nanowire comprising a p-doped end and an n-doped end        forming a a pn-junction in-between will be more easily        polarizable than an unipolar nanowire.        -   a. The p-doped end will become positively charged and the            n-doped end will be negatively charged when exposed to the            electrical field, and hence the nanowire will be oriented in            an unequivocal direction with the p-doped end pointing in            the direction of the electric field.        -   b. The same effect will apply to a unipolarly doped nanowire            where a Schottky diode is formed between the wire and its            seed particle.    -   3. Illumination of a nanowire containing a pn-junction will        induce a strong electric dipole with the same polarity as the        electric dipole formed by the electric field, greatly enhancing        the effect of the pn-junction itself (FIG. 1).    -   4. By illumination with light in different pre-determined        wavelength regions, nanowires having different band gaps can be        selectively aligned since wires that do not absorb the light        will have a much weaker dipole.

The pn-junction used for alignment may be a functional part when used ina device comprising the aligned nanowires. In addition the nanowires maycomprise additional functional sections that are not intentionally usedfor alignment.

How effectively a wire is aligned depends on its dimensions,composition, the external electric field, and whether or not a dipole isinduced, e.g., through illumination. The following general rules applyfor electric field alignment. The numbers given are based on asimplified theoretical model and should not be seen as limiting for thevalidity of the general statements. FIGS. 3-7 illustrate these rules.

-   -   1. In the simplified model,        -   a. the alignment energy due to an (induced) dipole (E_(d))            scales as the electric field to the first degree;        -   b. E_(d) scales as the square of the wire diameter but does            not depend on its length;        -   c. the alignment energy due to polarization of the wire            material (E_(p)) scales as the square of the electric field;        -   d. E_(p) scales as the cube of the wire aspect ratio (length            divided by diameter).    -   2. At low electric fields, typically below 100 V/cm, the        alignment is weak or non-existent, meaning that the alignment        energy is on the order of or below kT, which is the average        energy of Brownian rotation per wire; k is the Boltzmann        constant and T is the absolute temperature (300 K).    -   3. At high electric fields, typically above 10 kV/cm, all        elongated objects are aligned without regard to direction,        meaning that the alignment energy due to polarization (E_(p)) is        much greater than kT.    -   4. Thinner and longer objects are more readily aligned than wide        and short ones.    -   5. Under illumination or other sources of a potential difference        between the wire ends, i.e., a dipole, there exists a regime        where E_(d) is both greater than E_(p) and much greater than kT,        leading to orientation with a preferential direction; this        effect is greater for wider wires than for thinner ones (FIG.        7).

Deposition of the aligned nanowires can, by way of example, be done byone or a combination of the following:

-   -   1. Nanowires with a net charge will move in the electric field;        thus negatively charged nanowires will move downward, where a        substrate can be placed.    -   2. Random diffusion of uncharged dipoles, which is particularly        useful when the distance between the plate capacitors is small,        where        -   a. a sheath flow may be introduced to prevent deposition on            one side;        -   b. substrates may be placed on both sides of the nanowires            for deposition of oppositely aligned wires; and        -   c. the distance between electrodes can be made smaller than            the wire length, forcing the wires to touch the substrate,            either by moving the plates closer or by designing in a            constriction in the flow of nanowire-containing fluid.    -   3. Dipoles with or without net charge will move in an electric        field gradient, such that the wires are attracted towards higher        fields. This effect can be used both for deposition and for        classification of nanowires (FIG. 2).        -   a. Longer and thinner wires are subjected to a stronger            force due to the field gradient and will this move faster            toward the regions with higher fields.        -   b. The electric polarizability is different for different            materials.        -   c. For charged wires the gradient force can be balanced by a            force due to the charged, allowing further control of size            and material dependent classification.    -   4. On a patterned substrate electrode, the field gradient will        influence deposition; in combination with wavelength-selective        dipole generation, wires can be selectively placed according to        composition and/or size.    -   5. A structured electrode behind the substrate, e.g., a bed of        nails or array of ridges, will produce a pattern or dots or        stripes; an alternating potential will produce areas with        oppositely oriented wires.    -   6. Deposition may be locally enhanced or inhibited by patterns        of surface charge on the substrate; the deposition may be made        self-limiting when the charged areas are neutralized by the        charges from the deposited wires    -   7. Wires may be deposited through thermophoresis, wherein wires        suspended in a fluid will move towards lower temperature regions        in a thermal gradient, i.e., deposited on a cold wall and/or        repelled from a hot wall.    -   8. In the case of magnetic nanowires, they are easily collected        and controlled with magnetic fields. Charged nanoparticles        suspended in a flow of fluid (in effect generating an electric        current) are also affected by magnetic fields.    -   9. Intensely focused (laser) light and magnetic fields can in        some cases be used to trap nanowires locally, e.g., by the        optical tweezer or magnetic trap effects.    -   10. Apart from the above, other things such as ultrasound,        microwaves, etc., may be of use to inhibit or improve wire        deposition.

The electric field may be generated using two opposed electrodes, forexample two parallel plates, and applying a voltage between theelectrodes. The substrate used can also function as one of theelectrodes. In a continuous process the substrate, for example in theform of a web, foil or sheet may be fed between the electrodes (or overone of the electrodes if the substrate is used as one electrode) and apulsating or periodic voltage may be applied to the electrodes togenerate a varying electrical field and hence a varying orientation ofthe aligned nanowires.

According to the invention, nanowires can be deposited:

-   -   1. on any insulating, semiconducting or metallic substrate,        which is substantially flat;    -   2. on substrates in form of webs, foils, or sheets, preferably        in a roll-to-roll process, wherein the substrate is passed        through a point, stripe or region of deposition, much like a        printing press;    -   3. in the case of wires dispersed in a liquid, as a colloidal        suspension, the wire-containing liquid can be applied to the        substrate, and the wires be aligned        -   a. during drying/evaporation of the liquid        -   b. during or prior to solidification/polymerization of the            liquid    -   4. on substrates coated with a polymer, metal, liquid or other        material to enhance nanowire sticking and/or electrical        contacting, wherein        -   a. the sticky material may be very thin, so that only the            wire ends are stuck; or        -   b. the sticky material has a thickness of the same order as            the length of the nanowires; and        -   c. if the nanowires are dispersed in a liquid, the liquid            may comprise monomers to make the polymer on the substrate            gradually thicker, thereby encapsulating the nanowires;    -   5. on substrates patterned with extruding shapes, varying        stickiness, surface charges, etc., to enhance or locally inhibit        nanowire deposition;    -   6. on functional substrates, where the nanowires are intended to        enhance or modify the function;    -   7. in complex functional patterns of dots, stripes, etc.,        produced by a structured electrode behind the substrate or by a        structure in the (conducting) substrate itself, wherein        -   a. the polarity/direction of the nanowires may vary in,            e.g., stripes or checkerboard patterns; and        -   b. the nanowires are of different types and sorted by            inducing the electrical dipoles in a wavelength selective            way;    -   8. on both sides of a substrate, where the wires on either side        may be of different types and deposited by different means,        including any of the means described above;    -   9. in a continuous process, where contact or adhesion layers,        insulation oxides or polymers, etc., are deposited on the        substrate upstream or downstream the nanowires.

By the term “nanowire” in this patent application is meant any elongatedstructure with at least one dimension smaller than 1 μm. Typicalexamples include, but are not limited to:

-   -   1. semiconductor nanowires with a diameter of 50-500 nm and a        length of 1-10 μm, produced by, e.g., MOCVD growth, liquid        solution chemistry, gas phase growth. Typical materials are        III-V or III-N semiconductors (GaAs, InP, GaSb, GaInN and        related alloys), Silicon, Germanium, or II-VI semiconductors        (ZnO, ZnS, CdS, CdSe and related alloys);    -   2. metal nanowires produced by, e.g., electrodeposition in        anodized aluminum templates, whisker growth, gas phase growth.        Such nanowires may be made from magnetic, superconducting or        normal metals;    -   3. insulating, high bandgap semiconductor, or high-TC        superconductor nanowires, either manufactured or naturally        occurring;    -   4. carbon nanotubes; or    -   5. biological nanofibers, e.g., cellulose, proteins,        macromolecules and bacteria.

Devices to be produced with this method, however not limited to this:

-   -   1. Photovoltaic (PV) cells where the light-absorption and        charge-separation takes place in the nanowires alone, i.e. where        the nanowires contain pn-junctions or other rectifying        mechanisms. Such PV devices include a densely packed array of        nanowires, where the majority of the wires are vertically        aligned and connected with the same polarity. It would also        include a transparent contact either from the top or through a        transparent substrate, connecting all the wire ends in parallel.    -   2. Light emitting diodes (LEDs) where the light-emission and        charge recombination takes place in the nanowires alone, i.e.        where the nanowires contain pn-junctions or other rectifying        mechanisms. Such LED devices would include an array of        nanowires, which may be contacted either with the same polarity        or with random polarity. In the latter case, the structure would        function as a rectifier, suited for connection directly to AC        voltage. An LED array would also not need to be extremely dense,        if the objective is to produce large area light emitting        surfaces, for example ceramic tiles or even wallpaper.    -   3. PV cells where the nanowires constitute part of the        rectifying mechanism, and the other part(s) are in the        substrate; the substrate is, e.g., p-type and the wires n-type.        Such a PV device would be made from densely packed nanowires,        but here the up/down orientation of the wires is less important,        and consequently the processing simpler. In addition, the        density of the wires may be lower than in example 1 above, if        the carrier diffusion length in the substrate is large enough.    -   4. LEDs where the nanowires constitute part of the rectifying        mechanism, and the other part(s) are in the substrate; the        substrate is, e.g., p-type and the wires n-type. Just as in        examples 2 and 3 above, such an LED arrangement relaxes the need        for both alignment and density.    -   5. PV cells where the nanowires constitute part of the        rectifying mechanism, and the other part(s) are in the matrix        surrounding the wires after deposition, e.g., p-type wires in an        n-type conducting oxide or polymer. Such a PV device may be        fabricated on very simple substrates.    -   6. LEDs where the nanowires constitute part of the rectifying        mechanism, and the other part(s) are in the matrix surrounding        the wires after deposition, e.g., p-type wires in an n-type        conducting oxide or polymer. Just as in examples 2-5 above, such        an LED arrangement relaxes the need for both alignment and        density.    -   7. PV cells where the substrate itself is a photovoltaic cell,        and where the nanowires are designed to absorb light above or        below the bandgap of the substrate, thereby creating a tandem        photovoltaic cell. The nanowire part of the tandem cell may be        fabricated according to example 1, 3 or 5 above.    -   8. In all of the above examples of PV cells or LEDs, the        nanowires may be arranged in patterns, e.g. stripes, to allow        for spectrally resolved light absorption by means of a prism, to        achieve multi-junction PV functionality; or light emission with        adjustable color temperature where each stripe is contacted        separately.    -   9. In all of the above examples of PV cells or LEDs, the        nanowires also give a built-in nanostructuring of the top        surface, which may be advantageous for both light absorption in        the case of PV cells and for light emission in the case of LEDs.    -   10. Thermoelectric cells, wherein the one-dimensional character        of nanowires is used to improve the way thermal gradients can be        used to generate electrical power. Furthermore, by the        controlled deposition of separate fields of n-doped and p-doped        nanowires a Peltier element is formed, which can be used for        cooling or heating applications, in the conversion of electrical        power to thermal gradients.    -   11. Batteries where one or both electrodes are prepared by this        method and thus are constructed out of a nanowire structure. The        small diameter of the nanowires makes them insensitive to strain        and thus to changes in diameter that are accompanied with the        volume change during battery cycling.    -   12. Fuel cells, electrolysis or photolysis cells where the main        material is comprised out of nanowire structures which gives        them advantages over other materials in terms of insensitivity        to process-induced strain, surface-to-volume ratio, as well as        process fluid interaction with the structures.    -   13. Microelectronic or data storage devices may be formed in        nanowires which are deposited from the aerosol phase.    -   14. Templates for growth of functional material films on simple        substrates, where the aligned nanowires provide the crystal        structure for growth of, e.g., compound semiconductors on        Silicon or even on non-crystalline surfaces (metal, glass,        etc.).    -   15. A device for classifying nanowires according to size and/or        material, wherein a flow of charged or uncharged nanowires        passes through an electric field gradient. Longer, thinner and        more polarizable wires are subjected to stronger attractive        forces towards higher electric field regions, and can thus be        classified in a manner reminiscent of a mass spectrometer. For        charged wires, forces may be balanced for greater selectivity.    -   16. Nanocomposite materials, where nanowires are dispersed in,        e.g., a polymer matrix, leading to, e.g., enhanced mechanical        strength, increased electrical conductivity, improved gas        permeability properties, etc. Such nanocomposites may also be        sensitive to applied external forces and thus used as sensors.    -   17. Field emission electron sources where simple or        heterostructure-designed nanowires are deposited into arrays for        field-emission applications.    -   18. Antireflective or light filtering surfaces or smart windows.    -   19. Surfaces with enhanced mechanical sticking ability, due to        the gecko effect.    -   20. Surfaces with enhanced or reduced heat emission.    -   21. Chemical or biological sensors.

All references to upward, vertical, horizontal, lengthwise, etc. areintroduced for the ease of understanding only, and should not beconsidered as limiting to specific orientation. Further the dimensionsof the structures and axes in the drawings are not necessarily to scale.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the appended claims.

1. A method of aligning nanowires on a substrate during manufacture of ananowire structure, comprising the steps of: providing a population ofnanowires; illuminating at least a sub-population of nanowires withlight in pre-determined wavelength region so as to induce a firstelectric dipole in each nanowire, and applying an electric field (E)over the population of nanowires such that the nanowires become alignedalong the electric field (E), wherein each nanowire of the at least asub-population of the nanowires comprises one of i) a pn-junction, wherethe electric dipole is formed from an n-side to a p-side of thepn-junction; ii) a Schottky diode, where the electric dipole is formedfrom an n-side to a p-side of the Schottky diode; or iii) piezoelectricportions, where the electric dipole is formed by charge separation dueto a built-in piezoelectric field. 2-3. (canceled)
 4. The method ofclaim 1, wherein the population of nanowires comprises a plurality ofsub-populations of nanowires, the nanowires of each sub-populationhaving different bandgaps, and the method further comprises selectivelyilluminating the population of nanowires with light in differentwavelength regions in order to selectively align nanowires havingdifferent bandgaps.
 5. The method of claim 1, wherein the nanowires areprovided dispersed in a fluid.
 6. The method of claim 1, furthercomprising fixating of the nanowires in an aligned position.
 7. Themethod of claim 1, further comprising deposition of the nanowires on asubstrate.
 8. The method of claim 7, wherein each nanowire of the atleast a sub-population of nanowires carry a net charge, and theelectrical field (E) exerts a force on the nanowires carrying a netcharge, whereby the nanowires carrying a net charge migrate towards, andare deposited on, the substrate.
 9. The method of claim 7, wherein eachnanowire of the at least a sub-population of the nanowires is uncharged,and the electrical field (E) exerts a force on the uncharged nanowiresdue to the electrical dipole, whereby the uncharged nanowires migratetowards, and are deposited on, the substrate.
 10. The method of claim 7,wherein each nanowire of the at least a sub-population of the nanowiresis uncharged, and the nanowires being uncharged migrate towards, and aredeposited on, the substrate by means of diffusion.
 11. The method ofclaim 7, wherein the substrate comprises an adhesion layer.
 12. Themethod of claim 7, wherein the nanowires are deposited in a continuousprocess.
 13. The method of claim 12, wherein said population ofnanowires are repeatedly provided and deposited in a pre-determinedconfiguration along the substrate in a roll-to-roll process.
 14. Themethod of claim 7, further comprising depositing an insulating polymerto fill a space between the nanowires.
 15. The method of claim 7,further comprising depositing an electrode material that electricallyconnects to one end of the aligned nanowires opposed to the substrate.16. The method of claim 1, wherein the electric field is applied by afirst electrode and a second electrode arranged on opposite sides ofsaid population of nanowires and at least one of the electrodes istextured.
 17. The method of claim 1, wherein the nanowires are subjectedto a field gradient, whereby longer and thinner wires are subjected to astronger force due to the field gradient and will thus move fastertoward the regions with higher fields.
 18. A device for classifyingnanowires according to at least one of size and material, comprising adevice for providing an electric field gradient, and a device forpassing a flow of charged or uncharged nanowires through said fieldgradient, whereby longer, thinner and more polarizable nanowires aresubjected to stronger attractive forces towards higher electric fieldregions, and can thus be classified.
 19. A method for classifyingcharged or uncharged nanowires according to at least one of size andmaterial, comprising classifying the charged or uncharged nanowires bypassing a flow of the charged or uncharged nanowires through an electricfield gradient, whereby longer, thinner and more polarizable nanowiresare subjected to stronger attractive forces towards higher electricfield regions.
 20. The method of claim 1, wherein: said applied electricfield (E) is sufficiently strong to induce a second electric dipole inthe nanowires; a total induced electric dipole comprises an aggregate ofthe induced first and second electric dipoles; and each nanowire isoriented in a direction of the applied electric field (E) so that eithera positively or a negatively charged end of each nanowire according tothe first electric dipole is arranged proximal to a negatively chargedelectrode that contributes to generating the electric field (E).
 21. Themethod of claim 20, wherein, for each nanowire, whether the appliedelectric field (E) induces the second electric dipole in the nanowire isdependent on the nanowire's length and width.
 22. The method of claim 1,wherein: said applied electric field (E) is not sufficiently strong toinduce a second electric dipole in the nanowires; only the firstelectric dipole is present in the nanowires, each nanowire is orientedin a direction of the applied electric field (E) so that a positivelycharged end of each wire is arranged proximal to a negatively chargedelectrode that contributes to generating the electric field (E),